There has been a demand to monitor an OSNR in real time in each node in an optical network. For example, an OSNR of each wavelength channel is monitored in an optical network that transmits a wavelength division multiplexed (WDM) optical signal. Then, an optical path is flexibly controlled for each wavelength channel according to a result of monitoring an OSNR.
However, when an OSNR is monitored in each node, an expensive measurement device such as an optical spectrum analyzer is not desirable in order to reduce costs for establishing an optical network. In addition, in a WDM transmission system (such as a superchannel transmission system) in which frequency spacing between channels is narrow, it is difficult to measure a noise component using an optical spectrum analyzer.
Thus, there is a demand for a configuration and a method that make it possible to measure, without using expensive optical equipment, an OSNR in a WDM transmission system in which frequency spacing between channels is narrow. For example, a method for estimating an OSNR by electric signal processing has been proposed. In this method, a received optical signal is converted into an electric signal using a photo detector, and DC power and AC power are measured using this electric signal. Then, an OSNR is estimated according to the measured DC power and the measured AC power. Related technologies are disclosed in Japanese Laid-open Patent Publication No. 2016-208482, U.S. Pat. No. 6,433,864, and the following document. S. Oda et al. Optical performance monitoring for dynamic and flexible photonic networks, SPIE Photonics West 2015, 9388-13
In many optical networks, data is transmitted using a frame of a specified format. In general, the frame is configured by a payload and a header. Data is stored in the payload. Control information used to control a transmission of a data signal is stored in the header. A fixed pattern may be set in the header. The fixed pattern is configured by predetermined data or a predetermined bit string and used to establish a frame synchronization. Further, the fixed pattern may be used to measure a dispersion of an optical fiber link.
Polarization multiplexing has been put into practical use as a technology that increases a transmission capacity of an optical signal. The polarization multiplexing can transmit a signal using a set of polarizations that are orthogonal to each other. The set of polarizations that are orthogonal to each other may be referred to as an “X polarization” and a “Y polarization”.
In a polarization multiplexed optical transmission, frames that are transmitted using a set of polarizations are synchronized with each other, as illustrated in FIG. 1. Here, the fixed pattern described above is set in a position specified in advance in each frame. In other words, the fixed pattern of an X polarization and the fixed pattern of a Y polarization are concurrently transmitted. Here, when the same fixed pattern is transmitted by the X polarization and by the Y polarization, the polarization of a polarization multiplexed optical signal is fixed. On the other hand, the bit pattern in a payload portion is random, so the polarization state in the payload portion is random. As a result, when this optical signal is transmitted through an optical transmission link having a normal polarization-dependent loss (PDL) and the received optical signal is converted into an electric signal using a photo detector in an OSNR monitor, spike noise corresponding to the fixed pattern occurs in the electric signal because the optical powers in a fixed pattern portion and the payload portion are different from each other. Even if a fixed pattern in which an optical power in a fixed pattern portion is different from an optical power in a payload portion is used, spike noise may also occur. The spike noise corresponds to a state in which power varies greatly and instantaneously.
FIG. 2A illustrates an example of an output signal of a photo detector that converts a received optical signal into an electric signal. In this example, each frame carried by an optical signal includes a fixed pattern, as illustrated in FIG. 1. In this case, spike noise corresponding to the fixed pattern occurs in an output signal of a photo detector, as illustrated in FIG. 2A. However, spike noise occurs on one of the positive side and the negative side with respect to an average power level. In the example illustrated in FIG. 2A, spike noise only occurs on the negative side. The intensity of the spike noise is random, so low-frequency white noise occurs due to this spike noise. Further, it is difficult to remove positive/negative asymmetric spike noise with a random intensity using a low pass filter. Accordingly, it is difficult to measure DC power and AC power of a received optical signal accurately, and thus it is difficult to measure OSNR of the optical signal.
In FIG. 2B, a spectrum A represents a spectrum of an optical signal into which a fixed pattern has been inserted. In other words, the spectrum A represents a spectrum of the signal illustrated in FIG. 2A. A spectrum B represents a spectrum when only a payload has been transmitted. In other words, the spectrum B represents a spectrum of an optical signal into which a fixed pattern has not been inserted. Here, AC power is calculated by integrating a spectrum in a specified frequency range (such as 100 kHz to 500 kHz). However, as illustrated in FIG. 2B, an average power is increased when a fixed pattern is inserted into an optical signal. Further, an amount of increased average power due to a fixed pattern depends on a PDL. Thus, DC power when a payload is transmitted is not accurately measured, and an estimated OSNR value includes a large error.